Fasciola hepatica fluke infection is a parasitic disease whose primary reservoirs are ruminants. The infection is acquired through the ingestion of contaminated semi-aquatic plants, most commonly raw wild watercress. The incidence of human fascioliasis is estimated at approximately 300 cases per year in France; the last known outbreak in the region affected 7 people in 1981. The diagnosis of three cases of liver fluke infection between early March and mid-April 2002 was sufficiently unusual to prompt an alert, issued in late April, by the Tourcoing Hospital Center. Rapid coordination among stakeholders enabled the planning of an investigation into this outbreak. A confirmed case was defined as a person residing in the region who had tested serologically positive for fluke infection since early 2002. An active search for diagnosed cases was conducted through immunology laboratories serving the region. Additionally, screening for suspected cases was conducted through a survey of laboratories for blood tests suggestive of parasitic infection, as well as through raising awareness among clinicians and the general public. The exploratory survey of all patients provided information on the usual risk factors for distomatosis, particularly the consumption of watercress, lamb’s lettuce, and dandelion. A case-control study was conducted in May to confirm the hypotheses from the exploratory survey. It included all cases diagnosed by June 15 and two matched controls per case (age ± 5 years, sex, place of residence). Producers who provided the purchase locations were listed. Their crops were then inspected. RESULTS In total, the case search identified 18 patients. The exploratory survey noted the consumption of watercress purchased by 17 of them. In the case-control study, only the consumption of raw watercress (OR: 86.67, p<0.05) or cooked watercress (OR: 22.0, p=0.04) was significantly associated with the disease. The frequency of consumption was not associated with the disease. Watercress was most often purchased at supermarkets (15/17) and came from several producers; however, 15 cases had consumed products from the same watercress grower; for the 3 controls who had consumed it, the products came from other producers. One of this grower’s production sites was not protected from runoff from neighboring pastures. DISCUSSION The results of the case-control, food, and environmental investigations establish a link between the consumption of watercress from a specific grower and the outbreak. The outbreak affected few people relative to the customer base served by the supermarkets. However, the nonspecific nature of the symptoms suggests that not all cases have been diagnosed to date. This outbreak serves as a reminder that the risk of distomiasis has not disappeared and that it is necessary to maintain oversight of the hygiene standards in watercress farms. This investigation demonstrates the value, in the face of foodborne contamination, of collaboration among clinicians, biologists, epidemiologists, and administrative officials. (R.A.)